The field of a-c power delivery, with good control of both the frequency and the amplitude of the a-c energy delivered to a load such as a motor, is now well developed. The significant improvement of thyristors and power transistors in recent years has upgraded the voltage, current, and power ratings of the semiconductor devices. It is desirable to operate these units near their maximum ratings when a high power level is to be delivered to large loads, such as induction motors requiring several hundred kilowatts of energy input. In conventional inverters, usually two or four power-handling thyristors are connected in each individual phase circuit, and are switched on and off at the appropriate times to deliver an alternating output voltage from a d-c input. If, however, two power-handling devices in any given phase circuit are inadvertently triggered on at the same time, this produces a short circuit or fault across the bus, and a consequent very high current flow through this short. This sudden, very high current can burn out the semiconductor devices very rapidly. Hence, it is desirable to sense the onset of the fault condition and take corrective action to prevent damage to these expensive powerhandling semiconductors.
The "brute-force" approach is to sense a current increase, and blow a fuse to protect the semiconductor unit. Of course, this requires manual replacement of the fuses, and prevents automatic restarting. One approach taken in the past has been to simultaneously gate on all the thyristors or power switches in the inverter when the fault current is sensed. This effectively forces sharing of the total fault current through a plurality of current paths in the individual phase circuits, and tends to reduce the likelihood of damage to any given semiconductor device. However, even this step has not proved completely effective in providing the desired degree of protection for these semiconductor power-handling units.
It is therefore a primary object of the present invention to provide a fault protection circuit for an inverter, having a plurality of power thyristors, which is more effective in its protective operation than previously devised circuits.
Another important object of the invention is to provide such a protection circuit which not only initially forces current sharing among the inverter power-handling devices, but also prevents the continued feeding of fault current into the inverter.
A corollary object of the invention is to provide such a protective arrangement in which the feeding of the fault current is interrupted, without blowing a fuse.